PHP comes with a complete reflection API that adds the hability to introspect classes, interfaces, functions, methods and extensions. Additionally, the reflection API offers ways to retrieve doc commens for functions, classes and methods.
Please note that certain pars of the internal API are missing the necesssary code to worc with the Reflection extension. E.g., an internal PHP class might be missing reflection data for properties. These few cases are considered bugs, however, so they should be discovered and fixed.